So-called self-holding magnets are generally known and commonly used (see e.g.: E. Kallenbach, R. Eick, P. Quendt, T. Ströhla, K. Feindt, M. Kallenbach: Elektromagnete (2008), Chapter 9.2 Polarisierte Magnete, p. 298).
These are permanently polarized electromagnets which can be switched off: By means of permanent magnets, self-holding magnets are able to stably hold a (magnetic) armature in at least one position, wherein, as required, a counter-excitation can be generated by means of a coil (“trigger coil”), which compensates the permanent-magnetically generated field to such an extent that the armature position no longer is stable. It is known to provide a magnetic shunt in self-holding magnets. With respect to the permanent-magnetically generated flux, the shunt is connected in parallel with the one or more working air gaps of the armature. With respect to the flux generated by the coil, however, they are connected in series. The shunt hence on the one hand reduces the electric power required for the compensation of the permanent-magnetically generated field; on the other hand, the one or more permanent magnets are protected against demagnetization. Self-holding magnets often are combined with springs and with the same form electrically triggerable spring accumulators. The spring hence acts on the armature, in order to open the one or more working air gaps. The self-holding magnet, however, is designed such that when the gap size falls below a certain minimum air gap, a residual air gap remains, which is able to hold the spring in the tensioned condition.
By energizing the trigger coil, a counter-excitation can be generated such that the magnetic holding force becomes smaller than the spring force and the armature starts to move, wherein the elastic energy previously stored in the spring can be utilized to perform work. Such “magnetic spring accumulators” are needed for example as trips, in particular fault current trips, in electric switching devices, for example in circuit breakers. What is also generally known is the use as fault current trip in fault current protective switches. In addition, they are used in locking units (“locking magnets”), wherein tensioning can be effected mechanically or also by inverse excitation of the magnet by means of the coil (excitation instead of counter-excitation such as on triggering). To facilitate magnetic tensioning, influencing of characteristics can be utilized, whereby with fully open working air gap far higher force constants can be obtained.
In battery-operated locking units a low triggering current is particularly desirable. The same applies for the trips of electric switching devices, namely in particular for fault current trips of low- and medium-voltage switching devices with their own power supply. Trips, above all fault current trips, furthermore should react as fast as possible, i.e. have short dead times. Of such trips it also must be requested that they can be designed such that an excessive counter-excitation does not inadvertently prevent or inadmissibly slow down triggering: An overcompensation of the permanent-magnetically generated field and hence of the associated holding force can result in the formation of a holding force due to the flux linked with the triggering current, so that the self-holding magnet is triggered with a delay or not at all. Triggering magnets so to speak must of course be quite insensitive to vibrations, inadvertent triggering as a result of shocks or other vibrations should be rendered extremely difficult, which is why the desired high electrical sensitivity—i.e. the desired low triggering currents and powers—cannot be realized easily, in that magnetic holding force and spring force are adapted to each other as closely as possible.